Array sensor electronics

ABSTRACT

Circuits and methods for detecting and amplifying sensor and sensor array ( 102, 104 ) output signals are presented. According to some aspects, a nonlinear feedback loop containing a sensor element provides a nonlinear transformation that compensates for a corresponding nonlinear response of the sensor element thereby providing a linearized final output signal. In other aspects, idle sensors are coupled to a reference on non-idle sensor elements. Other aspects include sensor and amplification circuits which operate without traditional filtering or switching elements, such that a higher throughput is achieved and no settling time is required due to traditional transients, thus allowing for faster scanning of larger sensor arrays. Some embodiments of the present invention are directed to variable gap capacitive sensor arrays and the signal processing electronics.

RELATED APPLICATIONS

This application claims priority under 35 U.S.C. § 119(e) to co-pendingU.S. Provisional Patent Application Ser. No. 60/347,599, entitled“CAPACITIVE ARRAY SENSOR ELECTRONICS,” filed on Oct. 24, 2001. Thisapplication further relates to co-pending U.S. Provisional PatentApplication Ser. No. 60/343,714, entitled “TIME SPATIAL VISUALIZATION OFLINEAR ARRAY DATA,” filed on Oct. 24, 2001. This application alsorelates to a co-pending U.S. patent application Ser. No. 10/281,068entitled “VISUALIZATION OF VALUES OF A PHYSICAL PROPERTY DETECTED IN ANORGANISM OVER TIME,” filed on even date herewith. Each of theabove-cited applications is hereby incorporated by reference in itsentirety.

TECHNICAL FIELD

The present application relates to sensor systems such as capacitivearray sensors. More particularly, the present application relates tocircuits and systems for capturing, amplifying and processing signalsreceived from sensors and sensor arrays.

BACKGROUND

Sensors are devices that respond to a stimulus and produce a signalindicative of the stimulus' magnitude or other characteristic related tothe stimulus. The stimulus may be any physical quantity or parameterwhich can affect a sensor and is usually a measurable parameter oreffect. An array of sensors is a collection of individual sensors thatare positioned at discrete locations and are related to one another inat least some aspects.

Sensor arrays are used in applications such as imaging, and generallyinvolve a plurality of individual sensors placed in relation to oneanother such that an effectively larger sensor is formed by the array ofsensors. That is, when placing sensors at a plurality of discretelocations over a region of interest it is possible to make somedetermination or estimate of the stimulus over the entire region ofinterest. Extrapolation or interpolation can provide an estimate of themagnitude of the stimulus at a spot which does not itself contain adiscrete sensor. Furthermore, aggregate measures of the stimulus overthe entire region of interest or smaller regions within the region ofinterest may be obtained by averaging or other operations performed onsignals derived from individual sensors.

Applications in which such sensor arrays are useful include touch padsand distributed sensors that provide an indication of the location andmagnitude of a force or a pressure applied to a region of interest.

One type of sensor array is a capacitive sensor array. This arrayemploys a number of discrete capacitors distributed over a region of thearray which may be arranged in a pattern forming a grid. A grid ofsensors may comprise a plurality of capacitive sensors which may beindividually addressable or addressable in groups or in their entirety.Addressing specific sensors may be accomplished using multiplexerscoupled to the sensor array according to data or select signals onmultiplexer select lines to determine the individual sensors to bedriven or sampled. By driving a sensor it is meant the process ofgenerally exciting the sensor or energizing the sensor so as to producea measurement of the stimulus at the sensor. By sampling a sensor it ismeant receiving an output signal from the sensor to read or detect thesensor response to the stimulus. Thus, it is possible to selectivelymeasure a signal from a given capacitive sensor element located at aparticular column and row of the capacitive array. Multiplexers may beused to determine the particular row and column from which a measurementis desired.

Capacitive array sensors have been constructed of rows and columns ofconductive strips separated by a dielectric material. FIG. 1 illustratesa capacitive array 100 having conductive strips arranged along rows 102and columns 104. The rows 102 and columns 104 of the capacitive array100 may be separated by a flexible deformable material such as asilicone gel. The silicone gel (not shown) will deform in response topressure applied to a surface of the capacitive array 100. Thedeformation of the silicone gel or other flexible substance can causethe rows 102 and columns 104 of the capacitive array 100 to becomenearer or more distant to one another. Gap distance (d) is a factorwhich determines the capacitance of the capacitors 200 formed by ahintersection of the rows 102 and columns 104 of the capacitive array100. If the rows 102 and columns 104 of the capacitive array 100 arecoupled to electrical connections and to an external circuit, thecapacitance of each of the capacitors 200 formed by the intersection ofthe rows 102 and columns 104 can be measured individually.

A sensor array can be driven and sampled, one sensor at a time or ingroups, or in its entirety. By scanning the capacitive array 100 toobtain a signal or measurement from each of its individual elements 200,it is possible to form a real-time picture of the pressure applied tothe capacitive array 100.

FIG. 2 illustrates a single capacitive array element 200. The element200 is formed by an intersection of a row 102 and a column 104 of thecapacitive array 100. The figure illustrates a distance or gap (d) thatseparates the row 102 and column 104 conductive strips. The capacitanceof the capacitor 200 is generally proportional to the area formed by theintersection of the row 102 and column 104 divided by the distance d.Hence, changes in the distance d result in changes in the value of thecapacitor 200. The relationship between the capacitance of the capacitor200 and the stimulus, e.g., applied pressure, may be nonlinear for avariety of reasons. These reasons include the deformation response ofthe flexible deformable material, e.g., the silicone gel, as well asother physical and electrical responses of the variable gap capacitanceelement 200.

For large arrays, technical challenges arise in making fast measurementsor scans of the entire sensor array. For example, a sampling circuitsuch as a multiplexer that samples a selected row and column on which toperform a measurement would have to cycle through all rows and allcolumns (all elements of the array) at a rate sufficient to provide themeasurements as required by the specific application.

Nonlinear responses in the signals derived from the individualcapacitors and the stimulus, e.g., applied pressure, complicate thedesign of an overall sensor circuit. Furthermore, the measured signal istypically small compared to the driving signal which drives thecapacitive array. This results in a poor signal-to-noise ratio whenattempting to derive a useful modulation signal reflecting the quantitybeing measured. This is because noise becomes amplified as well as thesignal being measured when using simple signal amplification.

Traditional sensor circuits employ filters and switches that slowacquisition times by causing transients which need to decay betweenacquiring measurements from the various elements of an array. Forexample, in scanning a sensor array, a switch switches between theindividual sensors of a traditional array, causing a transient signal tooccur. Not only do transients slow the acquisition of a complete sensorarray scan, but they can affect the quality of a measurement of astimulus by introducing noise into sensed signals.

Furthermore, conventional sensor arrays contain considerable parasiticcapacitances between sensor elements and other parts of the circuit,such as ground. These parasitic capacitances can contaminate sensedsignals with noise and extraneous signal components and can requireextra filtering circuitry and processing time to compensate for theparasitic capacitance.

SUMMARY

Aspects of one embodiment of the present invention are directed to asensor system, comprising a sensor array having a plurality of sensorelements; at least one sensor element of the sensor array, havingaddressable connections designating the sensor element, that senses astimulus; and an amplifier, disposed in a feedback arrangement aroundthe sensor element, the amplifier receiving an input signalcorresponding to an output of the sensor element and providing an outputsignal that drives the sensor element.

Another embodiment comprises aspects directed to a method for measuringa stimulus on a sensor array, comprising sensing the stimulus using atleast one sensor element of the sensor array; generating a sensorelement output signal corresponding to the sensed stimulus; amplifyingthe sensor element output signal to generate an amplified signalrepresentative of the physical property; and feeding back the amplifiedsignal to drive the sensor element.

Still another embodiment comprises aspects directed to a method forlinearizing a non-linear sensor response, comprising sensing a stimulususing a sensor element; generating a sensor output signal correspondingto the stimulus; feeding back the sensor output signal to an input ofthe sensor through a non-linear transformer feedback loop correspondingto the non-linear sensor response.

Another embodiment of the invention comprises aspects directed to amethod for reducing parasitic capacitance in a capacitive sensor array,comprising selectively coupling at least one sensor element in thesensor array to a common potential during a time period in which thesensor element is idle.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, each similar component that is illustrated in variousfigures is represented by a like numeral, although this does notnecessarily signify that the components are identical. For purposes ofclarity, not every component may be labeled in every drawing. In thedrawings:

FIG. 1 illustrates an overview of a capacitive sensor array havingseveral rows and columns of conducting strips;

FIG. 2 illustrates an overview of a sensor element formed by the overlapof two conducting strips separated by a dielectric gap of size (d);

FIG. 3 illustrates a schematic representation of an exemplary system fordriving and sampling a sensor array and amplitude detection;

FIG. 4 illustrates a schematic representation of an exemplary system fordriving and sampling a sensor array, including a feedback loop;

FIG. 5 illustrates a schematic representation of an exemplary system fordriving and sampling a sensor array, including bias cancellation;

FIG. 6 illustrates an exemplary circuit for driving and sampling asensor array, including a feedback loop and bias cancellationcapability;

FIG. 7 illustrates a schematic representation of an exemplary secondstage amplification and rectification apparatus which can be used withsensor arrays; and

FIG. 8 illustrates an exemplary circuit according to the schematicrepresentation of FIG. 7.

DETAILED DESCRIPTION

The present description describes various aspects of preferredembodiments of the invention. Some aspects have been described in otherco-pending applications. Specifically, this application claims priorityunder 35 U.S.C. § 119(e) to co-pending U.S. Provisional PatentApplication Ser. No. 60/347,599, entitled “CAPACITIVE ARRAY SENSORELECTRONICS,” filed on Oct. 24, 2001. This application further relatesto co-pending U.S. Provisional Patent Application Ser. No. 60/343,714,entitled “TIME SPATIAL VISUALIZATION OF LINEAR ARRAY DATA,” filed onOct. 24, 2001. This application also relates to a co-pending U.S. patentapplication Ser. No. 10/281,068 entitled “VISUALIZATION OF VALUES OF APHYSICAL PROPERTY DETECTED IN AN ORGANISM OVER TIME,” filed on even dateherewith. Each of the above-cited applications is hereby incorporated byreference in its entirety.

The present invention is not limited in its application to the detailsof construction and the arrangement of components set forth in thefollowing detailed description of the preferred embodiment and drawings.Rather, the invention encompasses other embodiments and may be practicedand carried out in various ways. Also, the terminology used herein isfor the purpose of description and should not be regarded as limitingwhen used to describe aspects and embodiments of the invention. The useof “including,” “comprising,” or “having,” “containing,” etc., andvariations thereof are meant to be open-ended and encompass at least theitems listed thereafter.

FIG. 3 illustrates aspects of a capacitive array scanning system 250used to drive and sample a capacitive array 100. An oscillator 110provides an oscillator signal 300 to an input multiplexer (MUX) 120which selects from the elements of capacitive array 100 one or moreelements to be driven by at least the oscillator signal 300. Selectionof the one or more elements to be driven is made using data on the inputselect lines 130. By proper identification of the individual or group ofelements to be driven, a voltage or current may be supplied to selectedsensor elements 200 as described previously. Oscillator signal 300 maybe one of a plurality of signals driving elements 200.

Array input signal 302 provides a driving signal which is delivered toselected sensor elements 200. Once excited or driven by the array inputsignal 302, the capacitive array provides an array output signal 304. Anoutput multiplexer 121 receives the array output signal 304 and,depending on the data provided to output select lines 131, the outputmultiplexer 121 provides a selected signal 306. Again, the selectedsignal 306 may comprise one or more selected samples from the capacitivearray 100.

In some embodiments, the selected signal 306 be amplified by anamplifier 140. The amplifier 140 may be of any type, including analog ordigital types, that provides an amplified signal 308. According to someaspects, amplification of the selected signal 306 improves resolutionand accuracy of the measurement of the stimulus. An amplifier 140 mayprovide any gain, including gains greater than or less than unity andunity gains. The amplified signal 308 is therefore not constrained to bea signal having an amplitude greater than the selected signal 306.

The amplified signal 308 is detected by an amplitude detector 150, whichis typically matched to the range of expected amplitudes provided in theamplified signal 308. Note that some embodiments may employtransconductance amplifiers, energy converters, or other elements thatconvert one type of signal into another. For example, an electricalsignal such as a voltage may be converted into a corresponding opticalsignal.

The amplitude detector 150 may be any suitable amplitude detector thatmay detect the size, strength or amplitude of a signal. For example, theamplitude detector 150 may comprise a voltage-measuring circuit or acurrent-measuring circuit or a frequency measuring circuit, togetherdescribed herein as amplitude detectors for the sake of simplicity. Itshould be appreciated that the amplifier 140 may amplify any type ofcharacteristic of the selected signal 306 and that the amplified signal308 merely indicates that characteristic is enhanced to generally makeit simpler to read or measure the characteristic. As mentioned, theamplitude of an alternating current (AC) voltage signal may be aconvenient characteristic to amplify and measure using the amplitudedetector 150, however, the present invention is not so limited.

The amplitude detector 150 provides a detector output signal 310corresponding to the amplified signal and in turn corresponding to theselected signal obtained from the element or elements of the capacitivearray 100.

It is to be appreciated that FIG. 3 is merely one schematic embodimentof a capacitive array scanning system, and that various configurationsand equivalent circuits may be constructed accordingly. Other auxiliaryelements and circuit components, not described herein, may be used invarious applications and embodiments, depending on the need at hand. Forexample, filters such as high-pass filters, band-pass filters andlow-pass filters may reduce unwanted noise or provide other signalconditioning functions to the overall scanning system 250. Furthermore,signal processing techniques, implemented in hardware and/or softwaremay be used at one or more positions in the scanning system 250 to addor remove spectral characteristics or other features to or/from thevarious signals described above.

FIG. 4 illustrates an embodiment showing aspects of the presentinvention used in a capacitive array scanning system 255. Again, thescanning system is not limited in applicability to the capacitive arraysensors described herein, but is more generally applicable to othertypes of sensors as well. The scanning system 255 comprises a feedbackloop 320, which in some embodiments provides a linearizing function, aswill be described below.

An oscillator 110 provides an oscillator signal 300 to amplifier 141.The amplifier 141 amplifies the oscillator signal 300 and provides anamplified input signal to input multiplexer 120, sometimes referred toas a driving multiplexer (DMUX). The input multiplexer 120 receives theamplified input signal 301 as well as data on input select lines 130, asdescribed earlier. The array input signal 303 is provided by the inputmultiplexer 120 to selectively drive array element members 200 (notshown) of the capacitive array 100. The capacitive array 100 is drivenor excited selectively by the array input signal 303, as was describedabove. An array output signal 305 is provided from the capacitive array100 to an output multiplexer 121, sometimes referred to as a samplingmultiplexer (SMUX). The output multiplexer 121 samples the selectedarray elements according to the data on output sampling lines 131.

The output multiplexer 121 provides a selected signal 307 back to theamplifier 141. In this way, the amplifier 141, the capacitive array 100and other elements, are arranged in a feedback loop 320 by which theamplified input signal 301 and the selected signal 307 act to return aportion of the capacitive array's output to its input. In the embodimentof FIG. 4, input and output multiplexers, 120 and 121 respectively, arethe elements to which and from which the feedback loop 320 couples theamplifier 141 and the capacitive array 100.

The selected signal 307, once provided to amplifier 141 is alsoamplified and provided as an amplified output signal 309 to amplitudedetector 150. In some embodiments, signals 301 and 309 are the same orhave the same value. The amplitude detector 150 provides a detectoroutput signal 311, similar to that which was discussed with regard toFIG. 3. Again, the exact embodiment disclosed in FIG. 4 is not limiting,but may be adapted and substituted with one or more elements orauxiliary circuits, filters, amplifiers, etc., as called for by theapplication at hand.

It is also to be appreciated that the arrangement shown in the presentembodiment does not depict a physical layout of the elements of thescanning system. For instance, some elements of the scanning system 255may be implemented on remote circuits, as opposed to being implementedon the same circuit. Also, the entire scanning system 255 may beimplemented on a microchip or other integrated circuit that performs thescanning system's function. Furthermore, various functions of thescanning system 255 may be carried out in software or in firmware or inany combination of hardware and software suitable. Examples includedigital signal processing (DSP) hardware and/or software to performvarious functions, e.g. filtering and amplification andapplication-specific integrated circuits (ASICs).

The oscillator 110 provides an output voltage signal such as an ACwaveform. In some embodiments, the oscillator output signal 300substantially comprises a single frequency sinusoid. The oscillator 110may be free-running or may be used in a burst mode depending on theapplication. According to some aspects of the invention, the amplitudeand/or frequency of the oscillator output signal 300 may be altered toimprove measurement quality or scan rate or another operating parameterof the scanning system 255. Furthermore, the oscillator 110 may bereplaced by another suitable component that can provide a periodicdriving signal in a steady or pulsed or programmed mode. One example maybe to replace the oscillator 110 with a microcontroller or other digitalprocessing control unit that provides a signal substantially equivalentto that described as the oscillator output signal. The oscillator 110may be controlled by a microprocessor 400 that supplies a control signal411.

The input multiplexer 120 and the output multiplexer 121 may be ofsubstantially similar design in some aspects of the present invention.According to some embodiments, the multiplexers 120, 121 select a singlerow 102 and column 104 from the capacitive array 100. This selectiondesignates a single element 200 of the array 100. However, the input andoutput multiplexers 120, 121 may also be used to select multiple rowsand columns simultaneously.

Multiplexer designs having a fast settling time are preferred in someembodiments because they allow for fast switching between sensorelements at a high sampling rate, thus improving the overall bandwidthfor the scanning system 255.

The data on the input select lines 130 and the output select lines 131of the multiplexers 120 and 121 respectively, may be provided in anumber of ways. For example, the selection lines 130 and 131 may be setby a microcontroller or digital signal processing unit 400 and may alsobe set to increment automatically as would be done with a finite statemachine. Additionally, separate amplifiers may be included in either orboth of the multiplexers 120 and 121 at any of the inputs and outputs ofsaid multiplexers. Also, an amplifier may be constructed as part of themultiplexing scheme used by the multiplexers 120 and 121.

According to some aspects of the present invention, placing theamplifier 141 in the feedback loop 320 comprising the capacitive array100 and the multiplexers 120 and 121, allows for a higher gain and thusa higher sensitivity in the overall scanning system 255.

Amplifiers 140 and/or 141 may be implemented as described above and canalso include provisions for adjusting the gain and offset of saidamplifiers. Said gain and offset of amplifiers 140 and/or 141 may beprescribed on an element-to-element basis or as a single settingsuitable for all elements. That is, the amplification gain or schemeused for each individual sensor element may be individually tailored tothat element, or the gain may be held constant for the entire array 100.An approach combining the element-to-element setting and the singlesetting for all elements may be used as appropriate. A microcontrolleror digital signal processing unit 400 may control said offset and gaincorrections for best overall results using control signal 412.

FIG. 4 illustrates a microcontroller or digital signal processing unit400 which provides control signals 411 and 412 to the oscillator 110 andto the amplifier 141, respectively. The microcontroller 400 may beimplemented in hardware or in software or in a combination of hardwareand software, including a DSP component, as best suits the applicationat hand.

The amplitude detector 150 samples the selected signal 307 at a samplingrate which is typically greater than the array scan rate. According toone embodiment, if a 10 by 10 array is scanned at 100 Hz, then the scanrate is 10 kHz. In this embodiment, the amplitude detector 150 wouldcomplete individual measurements at a rate greater than 10 kHz.

According to some embodiments of the present invention, the amplitudedetector 150 is implemented using a rectifier that rectifies the outputof the output multiplexer 121, thereby acting as a nonlineartransformer. The nonlinear transformation achieved thereby may besubsequently augmented by integrating the resulting transformer outputsignal over an integer number of periods. In one embodiment theintegration is carried out over 10 cycles.

The nonlinear transformation mentioned above may comprise a functionthat creates a DC component in the signal proportional to the amplitudeof the sensed signal 305 or the selected signal 307. While not recitedherein for purposes of limitation, examples of such nonlineartransformation include full-wave or half-wave rectification,phase-corrected multiplication with the original driving AC signal, aswell as multiplying the output signal with itself to obtain the outputsignal squared. The amplitude detector 150 may further comprise aroot-mean-square (RMS) measuring circuit, a peak detector circuit anenvelope detector circuit, or an amplitude modulation circuit and alow-pass filter circuit. Amplitude detection could also be accomplishedin some embodiments by sampling the AC waveform using an analog todigital (A/D) converter and using a digital signal processor ormicrocontroller to compute the measured signal amplitude from thesampled data. As mentioned above, both digital and analog methods may beused for amplitude detection.

FIG. 5 illustrates an exemplary embodiment of a sensor system 265 havinga bias-canceling capability. In some embodiments it is advantageous tocancel a bias due to the driving oscillator signal so that a sensedsensor signal may be better amplified and discriminated from othersignals. The system shown in FIG. 5 is only an illustrative embodiment,and will be described below with respect to a capacitive array scanningsystem, however the system is not so limited and the sensor may be asensor other than a capacitor, as described previously.

An oscillator 110 provides an oscillator signal 300, as mentionedearlier. The oscillator signal 300 is amplified using amplifier 141 toproduce an amplified input signal 407. The amplified input signal 407 isprovided to input multiplexer 120 as previously described and the inputmultiplexer 120 subsequently provides a sensor input signal 401 based ondata at input select lines 130.

FIG. 5 illustrates a single sensor 200 rather than an entire sensorarray such as a capacitive array 100 discussed earlier. However, itshould be understood that one or more sensors or an array of suchsensors may receive an input signal such as an array input signal or, inthe present embodiment, a sensor input signal 401 from the inputmultiplexer 120.

As discussed previously, sensor 200 will provide a sensor output signal402 based on the sensor input signal 401 and corresponding to a sensedstimulus, such as force, pressure, etc. The sensor output signal 402 isreceived by an output multiplexer 121 which selects the particularsensor 200 from among a plurality of sensors in a sensor array such as acapacitive sensor array 100 (not shown). The output multiplexer 121selects the sensor output signal 402 on the basis of data presented onoutput select lines 131. The selected data 403 is provided from theoutput multiplexer 121 to the amplifier 141 and may form a feedback loopas previously described. The amplifier 141 provides an amplified outputsignal 404 which typically corresponds to the selected signal 403 andbeing amplified in its magnitude.

In the embodiment shown if FIG. 5, the amplified output signal 404 isfurther processed rather than merely being delivered to an amplitudedetector. Here, a second branch of the oscillator signal 300 is receivedby a phase-shifter 145 which provides a phase-shifted signal 405. Boththe amplified output signal 404 and the phase-shifted signal 405 areinput to an integrator 142 that substantially sums the two signals 404and 405.

The integrator 142 may integrate the signals 404 and 405 over severalcycles of the oscillator. The integrator 142 provides a time-integratedsignal 406 as an output. The integrator 142 may comprise a summingcircuit having an amplifier and an integrating feedback capacitance. Thesymbol at the output of the integrator 142 indicates that the sensingsystem 265 may comprise only a portion of a larger overall sensingsystem such as a capacitive array scanning system.

FIG. 6 illustrates a more detailed example of a sensor system 265similar to that described above in FIG. 5. In this embodiment, a chargeamplifier circuit is disclosed having a variable capacitance sensorCsense 200 which has a variable capacitance in response to appliedpressure on the sensing capacitor 200. A sine wave oscillator 110provides an oscillator signal 300 to the sensor 200 and to the rest ofthe charge amplifier circuit 265. The oscillator signal 300 passesthrough a reference capacitor Cref and is provided to the sensingcapacitor 200 through a drive multiplexer 120 (DMux). The sensingcapacitor 200 then provides a sensor output signal corresponding to thestimulus to the select multiplexer 121 (SMux). Amplifier U1 operates asamplifier 141 in the previous figures. Thus, a feedback loop between thesensing capacitor 200 and amplifier U1 going through multiplexers 120and 121 is created.

A bias cancellation circuit is provided by use of bias-cancelingcapacitor Ctune and gain-adjusting capacitor Gref. The bias-cancelingcapacitor is adjusted at the time of manufacture and is set in a waysuch as to cancel or reduce the AC amplitude of the driving carriersignal 300. Oscillator signal 300 passes through the branch containingcapacitor Cref to the inverting input of amplifier U1, while capacitorCtune operates as a filter to shift the phase of the oscillator signal300 at the output of the resistive digital to analog converter (RDAC).In this way capacitor Ctune and RDAC form a phase shifter as wasdescribed by block 145 in FIG. 5 above.

The output of amplifier U1 is provided through resistor R2 to anintegrator circuit formed by amplifier U2, capacitor C2 and resistor R3.The phase-shifted signal provided by the series combination of Ctune andRDAC is also provided to the inverting input of amplifier U2. Thenon-inverting input of amplifier U2 is coupled to ground through aresistor R4. Capacitor C2 and resistor R3 form a feedback impedancearound amplifier U2 thus integrating the input signals at the input ofamplifier U2. The integrator circuit, mentioned previously as block 142in FIG. 5, can also be considered a summing circuit which sums (a) theamplified output 404 and (b) the phase-shifted oscillator output signal405.

According to some aspects, the arrangement presented in FIG. 6linearizes the system's response in the presence of a nonlinear sensor200. The sensor loop including the feedback described previously makethe output of the sensing circuit correspond linearly to the stimulus(e.g., pressure capacitor gap, etc.) rather than the nonlinearrelationship traditionally provided in sensor circuits respondingdirectly (rather than inversely) to capacitance changes. Thus, thefeedback loop acts as a nonlinear transformer that counteracts thenonlinear behavior of the sensor 200 to yield a linearized output.

In some embodiments, the oscillator 110 is a single polarity excitationsource providing a stabilized sinusoidal waveform with a frequency in arange from 1 kHz to several megahurtz, for example from 50 kHz to 100kHz, depending on the rest capacitance of the sensing capacitor 200 andother design considerations such as a tradeoff between sensor range,sensitivity and linearity.

The capacitive sensor 200 or a sensor array 100, represented in FIG. 6by Csense, commonly also comprises parasitic capacitances to groundrepresented as Csg and Cdg. Parasitic capacitance Csg represents theparasitic capacitance from the sensor line to ground. Parasiticcapacitor Cdg represents the parasitic capacitance between the drivelines and ground. To reduce or eliminate parasitic capacitance, a sensor200 or a portion of a sensory array which is idle (not being driven orsampled) is coupled to ground to prevent parasitic capacitance effectsfrom influencing the measurements of the non-idle sensors. In someaspects this shunting to ground of the parasitic capacitance improvessystem throughput, linearity and bandwidth.

Capacitor Cref is normally selected to be a multiple of Csense, forexample Cref may have a value equal to three times the value of Csense.The ratio of capacitors Cref over Csense multiplied by the excitationamplitude of the oscillator determines the magnitude of the output ofamplifier U1. Thus, as Csense is increased the output of amplifier U1will decrease with a sensitivity dictated by the design of the sensingcapacitors Csense.

Other design considerations determine the nature of amplifier U 1characteristics that can be selected for the appropriate gain bandwidthand to minimize phase lag effects.

Furthermore, the driving and sensing lines to and from the sensor array100 may be individually shielded. In this way it is possible to preventcross-capacitance effects between the individual lines from impactingthe measured capacitance due to effects such as twisting or bending ofthe cable bundle running to or from the array 100.

It may be advantageous in some aspects to physically place any tunablefilter elements near the sensor elements 200 to provide common moderejection to environmental effects such as temperature changes.

As described previously, combining a phase-shifted signal through thecapacitor Ctune with the output of amplifier U1 in the integrator orsumming amplifier U2 makes the sensor's sensitivity positive, or inother words inverts the polarity, and allows for bias cancellation.Capacitor Ctune provides any necessary phase lag adjustment while theRDAC acts as a digital potentiometer to allow for precise bias trimmingfor each sensor element 200. The value of the RDAC is adjusted based ona measurement at the output of summing amplifier U2 with the sensor 200and its rest capacitance state. Such an adjustment is used to providesufficient trim for both the bias and gain of each sensor element 200having similar geometry or electrode surface area.

According to some aspects of the invention, the above-described sensorcircuit adjustment may simplify the calibration process. Calibration isnormally performed in an iterative process and is time consuming. Thepresent design may also reduce the intervals required betweencalibration procedures.

Resistor Rf is a feedback resistance and in some embodiments providesstability to the circuit. Rf may also be selected to optimize thesensitivity and linearity of the sensing circuit and in some embodimentsimprove the settling time to increase the throughput of large sensorarrays.

As mentioned earlier, the oscillator 110 may provide a variety ofoscillator signals 300. Such signals may be used as excitation waveformswhich can be sinusoidal as well as non-sinusoidal waveforms. Also, dualpolarity excitation is possible if its use is advantageous to aparticular circuit design.

An alternative embodiment allows for the deletion of any or all ofresistors R1, R4 and capacitors C1 and C2.

The output of the sensing circuit 265 FIG. 6 is provided in someembodiments to an input of an AC to DC conversion circuit such as arectifier and to a second stage amplification circuit before amplitudedetection is performed.

FIG. 7 illustrates an exemplary embodiment of a second stage circuit 275which may be employed downstream of the previously-described sensingcircuit 265. The sensing circuit 265 provides a sensing circuit outputsignal 406A, which may be the same as the integrated signal 406. Thissignal 406A contains information from the sensing element 200 indicativeof the measured stimulus as well as any remaining bias, noise and othersignal artifacts provided from the sensing circuit 265. The sensingcircuit output signal 406A is provided to a second stage amplifier 500which amplifies the signal corresponding to the desired measurement. Insome embodiments this signal 406A may comprise an AC sinusoidal signalhaving an amplitude of approximately 200 mV and a frequency ofapproximately 50-100 kHz.

An output signal 501 from the second stage amplifier 500 is provided toa rectifier 502. The rectifier 502 is constructed in any appropriate wayand converts an AC signal to a DC signal. A rectified signal 503 isprovided to a second stage integrator 504 which integrates signal 503over several cycles in time. According to some aspects, using the secondstage integrator 504 instead of a low pass filter stage providesimproved response time and avoids the need to wait until transientsdecay as is the case in low pass filter circuits. Integration in theintegrator 504 is carried out over approximately 10 cycles in someembodiments.

FIG. 8 illustrates one embodiment of a second stage amplificationcircuit 275 similar to that described in FIG. 7. A sensing circuitoutput signal 406A is received from a sensing circuit such as circuit265. The signal 406A represents an output from a charge amplifiercircuit which is oscillating at substantially the same frequency as theoscillator signal 300. The signal 406A is received from R5, the outputof which is coupled to ground through a capacitor C5 that acts as afilter.

A second stage adjustable gain amplifier U3A receives a non-invertinginput from signal 406A. The amplifier U3A is configured to provide anappropriate mid-band gain at the excitation frequency while maintainingapproximately unity gain at very low frequencies. The mid-band gain isadjustable via the gain resistive digital-to-analog converter (GRDAC).Capacitor C4 ensures that the gain near DC frequencies is low orapproximately unity.

Following the second stage amplification circuit 500 a signal 501 isprovided to a rectifier stage 502. The rectifier stage 502 comprises anamplifier U3B and performs precision full-wave rectification, especiallyat the excitation frequencies, including rectification for low inputamplitude levels.

The output of rectifier stage 502 is delivered on line 503 to averagingcircuit 504. Averaging circuit 504 comprises amplifier U3C and outputsan averaging circuit output 505 that is substantially a DC signalcorresponding to the stimulus delivered from the sensor 200. The overallcircuit of FIG. 8 represents an exemplary system for performing AC to DCconversion of the amplitude modulated signal from the charge amplifiercircuit 265 shown in FIG. 6. The circuits provided above perform theirfunction without requiring a switching element as in numerous existingsynchronous phase demodulator circuits.

Note that by reversing the polarity of the diodes D1 and D2 of therectifier stage 502 the rectifier stage 502 can be configured to providegain for negative or positive inputs and removing the RIO path canprovide for half-wave rectification.

The output of the second stage amplification circuit 275 may be an ACsignal 505 having an amplitude of 4VAC and a modulation of 2V,reflecting the measured signal and having a frequency of 50 to 100 kHz.Such a signal provides a better basis for measurement of variable, butsubstantially DC, modulation signals from the sensor elements 200.

Some embodiments of the present invention incorporate one or more of theabove-described aspects into a system which is coupled to an organismand detects physical parameters or properties of the organism.Time-series collection of biological data is one example of such anembodiment. For example, the circuits and methods described above may beused in conjunction with a catheter or other device, includingnon-invasive devices or minimally-invasive devices to collect biologicaldata on a human or animal patient. Pressure profiles measured within anorifice or a cavity in time and/or space can be collected forpresentation to a user or machine for storage, processing, or analysis.This may form a basis for a diagnosis of some medical condition or beused as a predictor for some other condition of the organism.

In one embodiment, a series of sensors arranged substantially in alinear form factor, are placed within a body cavity such as theesophagus and measure pressures in this cavity as a function of time.These esophageal pressures may then be displayed graphically on agraphical display, showing the physiological sequence of an action suchas coughing or swallowing.

Yet another aspect of the present invention permits the use of theabove-described systems and methods to obtain high-resolutionmeasurements of large regions of interest. Some embodiments of theinvention utilize several sensor array grids, each having its own sensordriving and sampling electronics, as described above, to carry outsimultaneous or sequential measurements. Multiplexing electronics areused in some embodiments to collect data from the multiple sensorarrays. These embodiments may in some regards be considered scalable orparallel implementations of the concepts described above.

As an example, consider the case where a large region of interest is tobe covered by sensors which collect data regarding a stimulus, e.g.pressure. The number of sensor elements will depend on the resolutionrequired (i.e. the grid spacing) and the overall area of the region ofinterest. If the grid spacing is tight (fine resolution) then the numberof sensor elements becomes large. In this case, interrogating or divingand sampling of each of the large number of sensors might entail cyclingthrough the sensors in the manner described above. For a large number ofsensors this can become time-consuming and slows down the sampling ratepossible for sampling each member of the array. To increase the samplingrate or to increase the possible resolution for a given sampling rate orto increase the overall area that can be sampled at some resolution at agiven sampling rate the region of interest may be broken into adjacent(tiled) sub-regions. Each of the sub-regions can be covered by a sensorarray as described earlier, and the output from each of the sub-regionscan be read by a circuit such as a multiplexer that switches betweeneach of the sub-regions in turn.

It can be appreciated that this technique can be used in an iterativefashion, thus nesting or scaling up or down the overall sensing systemso that an almost arbitrary area or resolution or sampling rate can beobtained, depending on the need. That is, in some aspects temporalperformance or spatial performance may be procured at the cost ofadditional hardware or processing sensor electronics.

Having thus described several aspects of at least one embodiment of thisinvention, it is to be appreciated that various alterations,modifications, and improvements may occur to those skilled in the art.Such alterations, modifications, and improvements are intended to bepart of this disclosure, and are intended to be within the spirit andscope of the invention. Accordingly, the present description anddrawings are given by way of example only.

1. A sensor system, comprising: a sensor array having a plurality ofsensor elements; an input multiplexer and an output multiplexerconfigured to select a sensor element; and an amplifier, disposed in afeedback arrangement around the sensor element and the input and outputmultiplexers, the amplifier receiving an input signal corresponding toan output of the sensor element and providing an output signal thatdrives the sensor element, wherein each sensor element has anindividually-controlled gain or an individually-controlled offset. 2.The system of claim 1, wherein the sensor element comprises a capacitivesensor element and the sensor array comprises a capacitive sensor array.3. The system of claim 1, wherein the sensor array comprises a first setof conductive strips crossing a second set of conductive strips to formeach of the plurality of sensor elements at an intersection of a stripin the first set and a strip in the second set, the sensor array adaptedand configured such that a stimulus alters a separation between a firstconductive strip of the first set and a second conductive strip of thesecond set.
 4. The system of claim 1, wherein the sensor array sensesany of a force, a weight, a pressure and a displacement.
 5. The systemof claim 1, further comprising an amplitude detector which receives anoutput signal from the amplifier.
 6. The system of claim 1, furthercomprising a nonlinear transformer which transforms a nonlinear outputof the amplifier into a corresponding substantially-linear signalrepresenting the stimulus.
 7. The system of claim 6, wherein thenonlinear transformer is a feedback loop comprising the sensor element.8. The system of claim 1, wherein the amplifier's output signal is oneof a plurality of signals which drive the sensor element.
 9. The systemof claim 1, further comprising a filtering circuit comprising adigitally-controlled integrator coupled to receive the output signal ofthe amplifier.
 10. The system of claim 1, wherein a tunable filterelement is coupled to the sensor element and wherein the tunable filterelement is placed in physical proximity to the sensor elements toprovide common mode rejection to environmental effects.
 11. The sensorsystem of claim 1, further comprising a resistor disposed in thefeedback arrangement in parallel with the selected sensor element.
 12. Asensor system, comprising: a sensor array having a plurality of sensorelements; at least one sensor element of the sensor array, havingaddressable connections designating the sensor element, that senses astimulus; an amplifier, disposed in a feedback arrangement around thesensor element, the amplifier receiving an input signal corresponding toan output of the sensor element and providing an output signal thatdrives the sensor element; a tunable filter, coupled to an oscillatorhaving an oscillator signal, which shifts a phase of the oscillatorsignal to provide a phase-shifted signal; and a summing circuit whichsums the amplifier output with the phase-shifted signal to substantiallycancel out oscillator bias in a portion of the sensor system.
 13. Thesystem of claim 1, further comprising a path from the sensor element toa common potential which is selectably coupled to the sensor element.14. The system of claim 1, wherein the sensor element is selectablycoupled to a ground potential when the sensor element is not beingdriven or sampled.
 15. The system of claim 1 wherein the sensor elementis coupled to an electrical line which is individually shielded fromelectromagnetic effects.
 16. The system of claim 1, further coupled toat least one other sensor system such that the system of claim 1 and theat least one other sensor system cover a combined region of interesthaving a greater area than an area covered by the system of claim 1 oran area covered by the at least one other sensor system.
 17. A methodfor measuring a stimulus on a sensor array, the sensor array comprisinga first set of conductive strips crossing a second set of conductivestrips to form a plurality of sensor elements, the method comprising,for each of a plurality of the sensor elements: selecting the sensorelement of the sensor array associated with a first conductive strip ofthe first set of conductive strips and a second conductive strip of thesecond set of conductive strips, the selecting comprising selecting thefirst conductive strip and the second conductive strip and an offsetbased on the selected sensor element; generating a sensor element outputsignal on the first conductive strip, the sensor element output signalbeing representative of a distance between the first conductive stripand the second conductive strip; amplifying the sensor element outputsignal to generate an amplified signal representative of a pressure onthe sensor element with the selected offset; and feeding back theamplified signal to the second conductive strip.
 18. The method of claim17, wherein: the sensor has a non-linear response; and feeding back theamplified signal comprises feeding back the amplified signal through anon-linear transformer feedback loop corresponding to the non-linearsensor response.
 19. The method of claim 18, wherein the non-linearsensor response is a non-linear response to a pressure applied to thesensor element.
 20. The method of claim 17, further comprisingselectively coupling at least one sensor element in the sensor array toa common potential during a time period in which the selected sensorelement is idle and at least one other sensor elements is being sampled.21. The method of claim 20, wherein the common potential is a groundpotential.
 22. A method for measuring a stimulus on a sensor arraycomposed of a first set of conductive strips crossing a second set ofconductive strips to form a plurality of sensor elements, comprising:selecting a sensor element of the sensor array associated with a firstconductive strip of the first set of conductive strips and a secondconductive strip of the second set of conductive strips; generating asensor element output signal on the first conductive strip; amplifyingthe sensor element output signal to generate an amplified signalrepresentative of a physical property; feeding back the amplified signalto the second conductive strip; providing an oscillating signal to drivethe sensor element, shifting a phase of the oscillating signal toprovide a phase-shifted signal; and summing the sensor element outputand the phase-shifted signal to substantially cancel out oscillator biasin a portion of the sensor system.
 23. A sensor system having an outputindicating a sensed property, comprising: a sensor array having aplurality of sensor elements; an input multiplexer and an outputmultiplexer configured to select a sensor element; an amplifier,disposed in a feedback arrangement around the sensor element and theinput and output multiplexers, the amplifier receiving an input signalcorresponding to an output of the sensor element and providing an outputsignal that drives the sensor element; and an amplitude detector havingan input and an output, the input being coupled to receive the outputsignal of the amplifier, the output of the amplitude detector beingcoupled to the output of the sensor system.
 24. The sensor system ofclaim 23, wherein each of the sensor elements comprises a capacitivesensor and the amplitude detector comprises a non-linear element. 25.The sensor system of claim 24, wherein the amplitude detector comprisesa rectifier.
 26. The sensor system of claim 23, further comprising: asumming circuit; and an integrator stage, wherein the input of theamplitude detector is coupled to receive the output signal of theamplifier through the summing circuit, and the output of the amplitudedetector is coupled to the output of the sensor system through theintegrator stage.
 27. The sensor system of claim 26, further comprisingan oscillator driving the selected sensor element, the oscillator alsobeing coupled to the input of the summing circuit through a phaseshifting circuit.
 28. The sensor system of claim 27, wherein: theamplifier is a first amplifier; and the sensor system further comprisesa second amplifier, the second amplifier being connected between thesumming circuit and the amplitude detector.
 29. The sensor system ofclaim 28, further comprising a resistor connected in parallel to aselected sensor element in the feedback arrangement.